The present invention related to improvements in thermal interfacing of electronic devices and the like in relation to heat sinks with which they are mounted for moderation of their operating temperatures and enhancement of their power-handling ratings, and, in one particular aspect, to novel and improved heat sinks prepared with elastomeric thermal-matching coatings cured in place in intimately-bonded relationship and exhibiting exposed compliant pre-cured dry surfaces for essentially voidless interfacing with devices from which heat must be extracted and dissipated efficiently.
As is well known in the art, applications of miniature semiconductor devices and microcircuit modules, and the like, may be severely limited in respect of the self-generated heat which they can successfully expel and prevent from building up as it is sought to have them operate at higher and higher powers. In many instances, costs and complexities and the environments of use prevent forced-fluid cooling systems from being used, and resort is then commonly had to more simple and economical expedients involving the attachment of simpler finned metal extrusions or stampings which serve to conduct and radiate heat away from thermally-vulnerable regions. In implementing efficient heat-sinking of either type, it is of special importance that the thermal impedance between a semiconductor or microcircuit unit and its associated heat-sink structure be kept to a minimum and that it be of uniformity which will prevent build-up of localized hot spots. Those characteristics are not likely to be achieved reliably and predictably, and to an optimum extent, by simply abutting some part of the heat-generating unit with complementary surfaces of its heat sink, largely because, despite superficial appearances, the interfacing between the respective mating surfaces will generally exhibit only a surprisingly small percentage of areas in actual sound contact. Such contact failure, with attendant poor transmission of heat, is caused in part by relatively gross surface imperfections, whose effects can be offset somewhat by subjecting the complementary surfaces to precise metal-forming or machining operations, and, in part, by essentially microscopic irregularities which will remain at the interfaces even if the cooperating parts are fashioned and finished with great care. When it becomes necessary to provide electrical insulation and high-quality dielectric characteristics between the mated parts, the heat-conduction problems are greatly compounded.
Among the prior practices which have been employed in efforts to improve the heat flow from semiconductor or like devices to their heat sinks is that of spreading amorphous oil or grease, such as silicone, between the joined surfaces. The messy aspects of use of such a filler, as well as the uses of insulating mica and varnish insulating layers, are referred to in U.S. Pat. Nos. 3,229,757 and 3,391,242. Thermal grease and/or powdered metal, contained by a film, has also been proposed to augment heat transfer, in U.S. Pat. No. 4,092,697, although the fabrication and handling of such small "pillows" obviously involves special problems as well. Cooling fins have been secured by way of epoxy cement loaded with powdered metal (U.S. Pat. No. 3,261,396), and large-area epoxy films have been said to insulate while yet transferring large quantities of heat (U.S. Pat. No. 3,611,046. In U.S. Reissue Pat. No. Re. 25,184, electrically non-conducting plastic coating material is filled with molybdenum disulphide to promote heat conduction, and, where electrical insulation is not essential, a dimpled malleable metal wafer may be interposed to increase transfer of heat (U.S. Pat. No. 4,151,547). Paper or cardboard carriers or pads have been proposed for convenient handling of a heat-transferring medium used to interface a semiconductor device and its mounting (for paraffin wax, in Canadian Pat. No. 600,823, issued June 28, 1960), and normally-solid waxy material which may be rubbed on to fill minute voids and thereby improve heat transfer has been described in U.S. Pat. No. 4,299,715. Pads which release a supply of silicone grease when compressed have been proposed as a means for reducing difficulties in the handling of silicone grease (U.S. Pat. No. 3,391,242), and beryllium oxide cement including a silicon rubber or other rubber-like plastic binder has been proposed as a means for securing devices to a heat-dissipating chassis while maintaining electrical insulation (U.S. Pat. No. 3,187,226). Dry silicone-rubber wafers have likewise been used to provide cushioned insulating mountings.